The present invention relates to a method and a system for mapping channels, typically uplink channels, into a convolutional encoded time division multiplex (TDM) in the downlink, applicable in transmission systems in general and in transmission via satellite in particular. An example of a transmission system via satellite is based on the use of a standard digital transmission format known as DVB (Digital Video Broadcasting) in which the TDM mode is employed as the transmission mode with convolutional code protection.
In a satellite communication system the transmission in the uplink direction is carried out normally using the transmission mode known as CW/TDMA (Continuous Wave/Time Division Multiple Access). In this type of transmission the data are transmitted in the uplink direction from a plurality of transmission sources, for example from a series of ground stations, up to the satellite; each transmission source using at least one transmission channel. On the other hand, said data are sent from the satellite in the downlink direction in TDM format at a constant bit rate and with a variable convolutional protection code.
The information bit rate is measured as the number of information bits transmitted per second. In the uplink direction this rate is different to the rate in the downlink direction in such a way that in a symmetrical transmission, that is when the total information flow in the uplink direction is equal to the same flow in the downlink direction, the following relation N*Ru=Rd is satisfied, where N is the number of uplink transmission channels, Ru is the uplink information bit rate and Rd is the downlink information bit rate.
However, in the transmission in the downlink direction the effect of the convolutional codes intervenes, said convolutional codes being some codes for protection against errors that add redundancy, the application of which is known in the art related with the invention.
In practice, the transmission rate in the downlink direction is maintained at a constant value, said value in turn depending on the values corresponding to the convolutional codes and the information bit rate in the downlink direction. Consequently, as shall be explained below, any change in said convolutional code shall necessarily change the value of Rd, which in turn shall compel the aforementioned relation to be changed. These changes in practice give rise to problems in the operation of the system since they can mean that the equipment hardware is made noticeably more complex, a problem that is of particularly significant importance in satellites, since it not only considerably increases the fabrication costs, but also produces problems of weight by increasing the satellite equipment payload.
A known solution for overcoming the aforementioned drawbacks proposes that in the case of a change in the convolutional codes, a consequence of which is a change in the downlink information bit rate Rd, a change is made in turn in the uplink information bit rate Ru, in relative proportion to said change in the downlink information bit rate Rd. The number of uplink transmission channels N shall be kept fixed and consequently the relation N*Ru=Rd shall not suffer variation.
The problem associated with this solution is that a change in the uplink information bit rate Ru would make it necessary to effect this same change in all the stations transmitting in the uplink direction. This problem can give rise to loss of synchronisation and shall in turn require new processes for acquisition of synchronisation and the subsequent resumption of normal transmission for all the users. Consequently, each change in code shall introduce necessarily an additional start-up time in the operation of each transmission station. Moreover, the known solution shall also make necessary the use of as many clocks or, in other words, working frequencies, as there are convolutional codes in the transmission stations and also in the receiving equipment.
For all the foregoing, it is desirable that the changes produced in the convolutional codes do not require changes in the uplink information bit rate.
The problems outlined hereabove are resolved by means of the method and the system for mapping uplink channels, in a time division multiplex (TDM) encoded and convolutional in the downlink direction, proposed by the present invention.
According to the invention, the changes in the convolutional codes in the downlink direction, and consequently in the information bit rate in the downlink direction, are offset by means of a change in the number of the uplink channels and not in the uplink information bit rate.
This objective is achieved by using selection criteria that are described further below in this description.
Thus an object of the invention is to provide a method for mapping channels into a convolutional encoded time division multiplex in the downlink, using at least one variable convolutional code rd, in a transmission system comprising a plurality of stations, each station transmitting in at least one channel, forming a plurality N of uplink channels being transmitted, at an uplink information bit rate Ru, towards a destination station, where said destination station transmits in at least one downlink channel at a downlink information bit rate Rd, in such a way that the relation Rd=N*Ru is satisfied, characterised in that the number of uplink channels N varies as a function of a change in said code rd, in such a way that N is an integer multiple of said variable convolutional code rd.
According to one aspect of the invention, said method is also characterised in that rd is expressed as rd=p/s, where p and s are whole numbers, and the value of N is expressed as N=mcm(s)*(p/s), where mcm(s) represents the least common multiple of the values s.
According to another aspect of the invention said method is characterised in that N*Ru=xcexa3Mj*Rui; where xcexa3Mj*Rui represents the sum of rates obtained as multiples of an initial rate Rui, and where Mj is a whole number, equal or different in each case, of multiplication of the initial rate.
Another object of the invention is to provide a system for mapping channels, into a convolutional encoded time division multiplex in the downlink, using at least one variable convolutional code rd, in a transmission system comprising a plurality of stations, each station prepared for transmitting in at least one channel, forming a plurality N of uplink channels being transmitted, at an uplink information bit rate Ru, towards a destination station, where said destination station is prepared for transmitting in at least one downlink channel at a downlink information bit rate Rd, in such a way that the relation Rd=N*Ru is satisfied, characterised in that the number of uplink channels N is an integer multiple of said variable convolutional code rd.
According to another aspect of the invention, said system is characterised in that it comprises a receiving antenna, at least one demultiplexer for demultiplexing the set of N channels received, at least one demodulator for demodulating the set of modulated channels, at least one multiplexer for multiplexing the N channels to be transmitted, at least one encoder of convolutional codes for applying convolutional codes to the multiplexed signal from the multiplexer and a transmitting antenna for transmitting a convolutionally encoded downlink channel.
According to an additional aspect of the invention, said system is characterised in that it also comprises at least one decoder for decoding the uplink channels.
These and other objects of the invention are described hereafter in greater detail with the assistance of the drawing attached.